There are two methods for driving a hybrid car according to the prior art: One is a series hybrid method wherein power is obtained by driving a generator by the driving speed of an engine as an internal combustion engine, and this power is used to drive the motor connected to the axle, then a car is driven by the driving force of the motor (Japanese Application Patent Laid-Open Publication No. Hei 08-298696, Japanese Application Patent Laid-Open Publication No. Hei 06-245322 and Official Gazette of U.S. Pat. No. 5,214,358, for example). The other is a parallel hybrid method wherein part of the driving force of the internal combustion engine is converted into electric power, and the remaining driving force is transmitted to the axle as driving force. Thus, the car is driven by both of motor driving force provided by generated electric power and axle driving force given by an internal combustion engine (Official Gazette of U.S. Pat. No. 5,081,365, for example).
According to the prior art, however, two motors and inverter circuits for driving these motors are necessary, and a new planetary gear mechanism must be installed. This requires a substantial improvement of a car, and resulting increase in costs cannot be avoided.
To solve this problem, a one-motor method is proposed as disclosed in the Official Gazette of Japanese Application Patent Laid-Open Publication No. Hei 07-298696. According to this method, an electric rotating machine is directly coupled to the crank shaft of an internal combustion engine, and driving and power generation are performed by one electric rotating machine through switching of operation mode. This method is more advantageous than the two-motor method discussed above in that the cost is lower and this feature can be added onto the current car.
For both the one-motor method and two-motor method, the following motor generator is used as an electric rotating machine; (1) a synchronous magnet type motor generator with a permanent magnet mounted on the rotor, (2) a synchronous motor generator of jaw type magnetic pole based on the same principle as that of an alternator as a car generator, or (3) a squirrel cage type induction motor generator with a secondary conductor of squirrel cage type installed on the rotor. Inverter control is adopted as follows: When the internal combustion engine is started, the output voltage of a 42-volt battery is adjusted by controlling the voltage, current and frequency by an inverter, and the motor generator is driven in the motor operation mode. After the internal combustion engine has started, the motor generator is driven in power generation mode so that power generation voltage will reach the level of battery charging voltage.
In the meantime, Japanese Application Patent Laid-Open Publication No. Hei 11-220812 discloses the method wherein a step up/down chopper is installed between the battery and inverter to ensure that d. c. input voltage of the inverter or the like is kept almost constant.
[Problems to be Solved by the Invention]
The following problems are found in the one-motor type motor generator used as a motor generator in a hybrid car:                (1) Compatibility must be ensured between high torque characteristic in the low speed area at the start of the internal combustion engine, and high output power generation characteristic capable of getting a high power generation current in the range from idling speed to high speed.        
(2) There is a relation of 1 to 10 or more between the rotational speed (about 700 rpm) for generating torque (maximum torque generated by the motor) required at the start of internal combustion engine in (1) and motor rotational speed (6000 rpm or more) at the maximum permissible rotational speed of the internal combustion engine.
(3) There is a shortage of the assist torque for promoting the torque of internal combustion engine at the rotational speed equal to or greater than the rotational speed when the internal combustion engine starts.
(4) The motor generator mounted on the car operates in the motor operation mode at the time of startup and generates power in the generator mode. A battery which is charged or discharged within the voltage fluctuation range with reference to a certain voltage is used as a power source. So the battery may be damaged in the worst case if it is charged with the voltage far exceeding the battery charging voltage, for example, at a high speed of the internal combustion engine.
The above problems must be solved when any one of the above-mentioned motor generators is used. Generally, when the motor of the motor generator is operated, rotational speed N is proportional to the applied voltage V and is inversely proportional to field magnetic flux φ.
Torque τ is proportional to the product of motor current Im and field magnetic flux φ. The counter electromotive force in motor operation and power generation voltage in generator mode operation are proportional to the product of rotational speed N and field magnetic field φ. Accordingly, the system must be configured to ensure that any motor generator can provide the required torque and generation power even when the rotational speed range is wide.
Generally current phase is controlled in such a way that strong field current component is obtained when a high torque is required at a low rotational speed, and weak field current component is obtained at a high rotational speed to reduce counter electromotive force.
However, when the motor generator is operated in the generation mode, generation is performed in the range from the idling speed (about 700 rpm) of the internal combustion engine to the maximum rotational speed (6000 rpm or more) of the internal combustion engine. So at a high rotational speed, power generation voltage becomes excessive in the method of adjusting the current phase of the stator winding. As a result, there is a shortage of weak field current component, and it is difficult to reach agreement with battery charging voltage. Further, when the internal combustion engine is started by motor operation, the starting current of the motor is excessive, current capacity of the switching element in the main circuit of the inverter becomes excessive, and this gives rise to problems.
The object of the present invention is to solve the above-mentioned problems and to provide a motor generator and control method thereof in a battery-mounted hybrid car wherein the motor generator connected to an internal combustion engine is operated in the motor operation mode or generator operation mode within the range from high to low speeds. This motor generator and control method thereof is further characterized in that stable power torque characteristic and power generation characteristic can be obtained and highly efficient control is ensured.